Many heavy-duty vehicle engines that run on liquid natural gas (LNG) require that the intake vapor pressure of the natural gas into the fuel tank be at a certain level, such as around 100 psig. In most markets, LNG is saturated, or heat is added, to a point at which its vapor pressure is roughly equal to the pressure required by the use device (such as a vehicle). This process of building saturation pressure is typically performed at LNG fueling stations. However, in some markets, the saturation of the fuel before transferring it from the LNG fueling stations to the vehicle storage tank is not performed. Thus, the storage tank in the use device (for example, vehicle) ends up being filled with LNG well below the desired pressure. This can lead to reduced vehicle performance.
One proposed method for building storage tank pressure is to utilize a pressure building circuit that is common on many stationary cryogenic cylinders. These circuits function by utilizing gravity to feed liquid cryogen into an ambient vaporizer. Upon vaporization of the liquid, its volume expands and the vapor is routed to a vapor space above the cryogen. This builds a head of vapor pressure above the liquid in the tank. However, there are two distinct problems with this type of circuit for LNG vehicle tanks. First, as most LNG vehicle tanks are mounted horizontally, there is small liquid head pressure (compared to a vertical tank) to force liquid into the ambient vaporizer. Second, since LNG vehicle tanks are used in mobile applications, any vapor pressure that is built above the liquid phase will quickly collapse as soon as the vehicle is in motion and the liquid and vapor phases mix. It may take several hours or more than a day to add enough heat in this fashion to fully saturate the bulk of LNG in the tank.
Another proposed solution is referenced in U.S. Pat. No. 5,163,409 whereby compressed natural gas (CNG) is used to add vapor pressure above LNG to deliver the fuel at an elevated pressure. However, this solution requires a second tank for CNG be mounted on the vehicle, which would undesirably add weight and occupy space on the vehicle chassis.